Transformers are available in 300, 600, 900, 1200, and 1500 watts. The size transformer you need will depend on the number of lights, wattage of lights, and length of wire run to the lights. Note that transformer sizing is not as simple as adding together the wattage of all the lights on a system to determine the correct transformer size. The length of wire runs will also factor into what size you will need since the resistance in the wire acts as additional wattage or “load”. To best determine correct transformer size, use the Low Voltage Landscape Lighting Calculator. Once you enter the wire distance, wire size, and wattage for each run, the calculator will determine the transformer size for you.